In recent years, studies to reduce the size of particles have progressed. In particular, intensive study has been conducted to reduce the particles into nanometer sized, for example, in the range of 10 to 100 nm, which can hardly be realized by methods of pulverization and others. Further, attempts have been made not only to provide particles whose particle sizes are reduced to be of the order of nanometers, but also to form them having monodispersity.
Such fine particles in the nanometer size, whose size is between common bulk particles and molecules or atoms, are said to have potentially unexpectedly new properties. It is also possible to stabilize properties of such nanoparticles by raising the monodispersity thereof. Thus, the nanoparticles are finding applications in various fields, and various studies are under progress increasingly, in a wide range of fields including biochemistry, new material, electronic element, light-emitting display device, printing, medicine and others.
In particular, organic nanoparticles made of an organic compound involve great potential as a functional material, because the organic compounds, per se, can be modified diversely. For example, polyimide has been utilized in various fields because of, for example, the following reasons: polyimide is a chemically and mechanically stable material owing to, for example, its heat resistance, solvent resistance, and mechanical characteristics, and is excellent in electrical insulating property. In addition, polyimides are finding application in a further wider range of industrial fields, by forming them as fine particles and combining their properties and shape of polyimides. For example, use of polyimide formed as fine particles as an additive to the powder toner for image formation was proposed (Patent Document 1).
Further, among the organic nanoparticles, organic pigments are used in such applications as painting, a printing ink, an electrophotographic toner, an inkjet ink, and a color filter, and thus these organic pigments are now important compounds essential for our everyday life. Particularly, organic pigments are demanded in high-performance with practical importance including pigments for an inkjet ink and a color filter.
Dyes have been used as the colorants for inkjet inks, but pigments are employed recently for solving problems of the dyes in water resistance and light resistance. Images obtained by using a pigment ink have an advantage that they are superior in light resistance and water resistance to the images formed by using a dye-based ink. However, it is difficult to give fine particles having excellent monodispersity and having nanometer size, so that the pigment particles can hardly penetrate into the pores on paper surface. As a result, such an image has a problem that the adhesiveness thereof to paper is weaker.
Further, the increase in the number of pixels of a digital camera, there is increased need for reduction in thickness of the color filter for use in optical elements such as a CCD sensor and a display device. Organic pigments have been used in color filters whose thickness depends significantly on the particle diameter of the organic pigment to be used therein, and hence it is needed to produce fine particles in a nanometer sized, with having stability in a monodispersed state.
As for production methods of organic particles, studies are made on, for example, a gas-phase method (a method of sublimating a sample under inert gas atmosphere and depositing particles on a substrate), a liquid-phase method (a reprecipitation method for obtaining fine particles, for example, by injecting a sample that has been dissolved in a good solvent, into a poor solvent of which the stirring condition and the temperature are controlled), and a laser-ablation method (a method of reducing the size of particles by laser-ablation to a sample dispersed in a solution with laser irradiated thereto). There are also reports on an attempt of preparation of monodispersed particles having a desired particle size by those methods. Of those, the liquid-phase method has been attracting attention, since it is a method of producing organic particles excellent in its simplicity and productivity (see Patent Document 2, Patent Document 3, and the like). The crystalline form and the nature of the surface of each of organic particles produced by the liquid-phase method can be controlled by adjusting conditions under which the particles are formed in accordance with, for example, the kind of solvent, the rate of injection, and temperature. Patent Document 3 describes an example in which the crystalline form of a quinacridone pigment is adjusted in accordance with a poor solvent kind.
With regard to an improvement in dispersibility of particles, an organic pigment has been conventionally dispersed on an industrial scale by using various dispersing machines (such as a roll mill, a ball mill, and an attritor). In this case, however, a particle in the pigment is reduced in size, with the result that the viscosity of the pigment dispersion may increase. The increase in viscosity makes it difficult to take the pigment dispersion out of a dispersing machine, makes it impossible to transfer the pigment dispersion through a pipeline, and, furthermore, causes gelling of the dispersion during storage so that the pigment dispersion cannot be used. A dispersing agent that aids the dispersion, or a polymer that stabilizes the dispersion has been added for solving them, but it cannot attain a sufficient effect (see, for example, Non-Patent Document 1).
In an organic-pigment dispersion for a color filter, in order to improve the dispersibility, a polymer or a pigment-dispersing agent capable of imparting both alkali developability and dispersion stability needed for the production of a color filter is added (see, for example, Patent Document 4). However, such methods have not satisfied the demand yet because of, for example, the following reasons: such methods require a long period of time for dispersing, and involves an increase in viscosity of the dispersion.
In addition, an example in which dispersibility is improved by using pigment particles prepared by the liquid-phase method has been reported. Patent Document 5 describes an example in which pigment particles in a water dispersed state is prepared by the liquid-phase method. However, this method is a method of providing pigment particles in an aqueous dispersed state, and the document describes nothing about a method of providing pigment particles in an organic solvent dispersed state.
Patent Document 6 describes a method of forming the pigment by dissolving a pigment in a basic compound and/or a basic solution and adding a liquid of a neutral compound and/or a liquid of an acidic compound, or a neutral liquid and/or an acidic liquid. However, organic pigment particles obtained by the method have large primary particle diameters, and then the method has not sufficiently satisfied a demand for a reduction in particle size.
[Patent Document 1] JP-A-11-237760 (“JP-A” means unexamined published Japanese patent application)
[Patent Document 2] JP-A-6-79168
[Patent Document 3] JP-A-2004-91560
[Patent Document 4] JP-A-2000-239554
[Patent Document 5] JP-A-2004-43776
[Patent Document 6] JP-A-2004-123853
[Non-Patent Document 1] Pigment dispersion technique-surface treatment and how to use dispersing agent, and evaluation for dispersibility-, TECHNICAL INFORMATION INSTITUTE CO., LTD, 1999